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Artificial Upwelling Maintains Favorable
Summer Environment For Farmed Oysters
Darien D. Mizuta, Ph.D.
Fisheries and Environmental
Oceanography Laboratory
Field Science Education
and Research Center
Kyoto University
Kyoto, Japan
[email protected]
Akihide Kasai, Ph.D.
Fisheries and Environmental
Oceanography Laboratory
Field Science Education
and Research Center
Kyoto University
Hitoshi Yamaguchi, Ph.D.
Copyright © 2015, Global Aquaculture Alliance. Do not reproduce without permission.
A circular configuration of tubing pumped water up from the lower depths of the bay,
resulting in waves on the surface. The aeration center is located at the yellow buoy.
Summary:
The summer season poses threats
for oyster aquaculture worldwide.
In addition to high mortality,
poor oyster quality and health –
especially in enclosed bays – are
often attributed to water stratification, high temperatures and
hypoxia. An artificial upwelling
system was installed at the bottom of an enclosed bay in Nagasaki, Japan to assist oyster culture
during unfavorable summer conditions. The aeration decreased
temperature and water column
stratification, increased dissolvedoxygen levels, and promoted the
quality and quantity of phytoplankton biomass.
Summer mortality of oysters is a global
problem that affects production at commercial farms. Mortalities result from the
physiological stress and depletion of
energy reserves induced by exposure to
stressful environmental conditions. Common seasonal stressors in enclosed bays,
where most aquafarms are located, include
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high water temperatures, eutrophic conditions, fouling organisms and hypoxia,
mainly in bottom layers due to thermal
stratification of the water column.
Oysters that survive the summer
period are often stressed and weak, which
usually influences their size, taste of meat
and overall quality. Therefore, there is a
strong desire for effective mitigative technologies and managements that can overcome summer-related problems during
the growout phase to guarantee highquality oysters.
Artificial Upwelling
Artificial aeration upwelling, which
consists of pushing colder, nutrient-rich
bottom water toward surface layers, promotes circulation in the water column
and is considered an advantageous
method to increase the availability of
phytoplankton for filter-feeding bivalves.
Nevertheless, only limited research has
assessed the possible contributions of the
technique to the improvement of temperature, oxygen and bivalve conditions.
To help fill this knowledge gap, the
authors designed two aeration systems in
cooperation with Ohishi Construction
Co. Ltd. and installed them in a sum-
global aquaculture advocate
Nagasaki Prefectural Institute
for Environmental Research
and Public Health
Nagasaki, Japan
Hideaki Nakata, Ph.D.
Graduate School of Fisheries Science
and Environmental Studies
Nagasaki University
Nagasaki, Japan
mer-stratified, hypoxic bay. The main
goal of the study was to check for the
possible environmental improvements
and related effects on farmed oysters.
Experimental Setup
Collaborative work between research
institutes and local farmers of the commercially important Ohmura Bay in
Nagasaki, Japan, investigated the effects
of aeration on the environment and on
farmed Pacific oysters, Crassostrea gigas,
in the summers of 2011 and 2012.
The aeration was performed from the
bottom at two locations: a relatively deep
area at 20 m depth in the center of the
bay, where bottom layer hypoxic water is
first formed, and at 5.5 m depth under an
oyster farm whose cages were stocked
with juveniles. Air was supplied by tubes
connected to compressors at a rate of
1.26 m3/minute for the deeper area and
0.15 m3/minute for the farming area.
In both study areas, the sampling stations were located at the aeration center
and at increasing distances from it. Water
samplings were carried out at 30-day
Artificial upwelling resulted in greater growth and
better condition of the
oysters, and improved survival, as well.
150
Aeration Center
Dissolved Oxygen (%)
Far From Aeration
100
Figure 1. Differences in dissolvedoxygen concentration at the bottom
layer.
50
0
July 1
August 1
August 31
intervals for environmental data, and oysters were sampled at the farming area.
Data were compared among stations to
check differences induced by the aeration.
Environmental Improvements
The effects within 30 m of the aeration were noted in changes in temperature, oxygen concentration and availability of food.
Decreased water temperature and stratification. Water temperature was significantly decreased by the upwelling of cold
bottom waters. Temperature stratification
indices measured by temperature recorders
were an average 0.42° C lower at the aeration point than the furthest station.
Restricted increase in oxygen concentration. Although restricted only to the beginning of summer and autumn, the oxygen
levels were increased in the bottom layer
near the aeration (Figure 1). Although the
aeration decreased the oxygen concentration in upper layers above 4-m depth, during the period of bottom hypoxia, the dis-
October 1
October 31
solved-oxygen levels were still above 3
mg/L, which is considered the hypoxic
limit. New aeration rates and designs
should be tested to guarantee the positive
effect of the aeration throughout summer.
Increased food availability and food
quality. The diatom biomass was almost
twofold higher at the aeration center than
at stations located 60 and 180 m from it
(average values: 162,875 cells/L, 139,433/L
and 85,133 cells/L, respectively).
The dominant phytoplankton species
changed from dinoflagellates, regarded as
poor-quality food for bivalves, to higherquality food diatoms. Food quality was
also assessed by the carbon:nitrogen
(C:N) ratios of local suspended particulate organic matter, which indicated progressively higher nutritive rates for the
suspended matter as their value
decreased. At the aeration point, C:N
ratios were lower (Figure 2).
Oyster Condition
Oyster overall condition improved,
which was proved not only by the shell
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March/April 2015
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Carbon:Nitrogen Ratio
Depth (m)
5678910
0
-5
Far From Aeration
Aeration Center
-10
Maximum
Minimum
-15
Figure 2. Mean carbon:
nitrogen ratios
for suspended particulate
organic matter at two
locations. Maximum
and minimum values
at the aeration center
are also shown.
Condition Index
-20
120
100
2011
2012
80
Figure 3. Condition
index of oysters in
relation to the distance
from the aeration
point in 2011 and
2012.
60
40
20
0
0
60
180
Distance From Aeration Point
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height growth of almost 9.7 mm/month,
but also by higher condition index (C.I.)
which is a measure of health and determined by the formula C.I. = (dry meat
weight/dry shell weight) x 1,000 (Figure
3). Total survival was not affected in deep
layers, but slightly increased at the surface
at the aeration spot (42%), when compared to the survival percentages at other
locations (30%).
Perspectives
Artificial upwellings were shown to
contribute to improvements in farming
sites and cultured products during unfavorable periods. However, it is clear the
technology should be the subject of future
investigations, with assessments of different aeration rates and designs related to
each targeted species and possibly under
an approach considering global warming.
The costs of aeration were within the
usual affordable prices for the fisheries
cooperatives in the studied area and
resulted in cost-effective positive outcomes. Customized systems, nevertheless,
will require strategic planning for each
objective in order to provide a basis for
decision makers about the benefits of the
implementation of such systems for profitable mariculture.